A rotary encoder is a device which measures the angular position of a shaft around a rotational axis of the encoder. There are two types of rotary encoders: incremental, which determine the relative change in position, or motion, of the shaft; and absolute, which determine the absolute position of the shaft around the rotational axis of the encoder. Absolute rotary encoders are widely used in a range of applications and technical fields such as robotics, medical devices, telescopes and CNC machines.
There are many types of absolute rotary encoder devices (e.g. magnetic, capacitive, mechanical, and optical). For applications requiring a relatively high resolution encoder, optical and magnetic rotary encoders are the most widely used devices. Optical rotary encoders are generally high resolution and high accuracy devices, but tend to be expensive. In addition to their cost, optical encoders are not suited to many applications because of their lack of robustness; they must be precisely aligned and are particularly sensitive to contaminants such as dirt or moisture, which can cause optical errors and lead to incorrect position readings. Magnetic encoders have a lower resolution and accuracy than optical encoders, but are cheaper and more robust. Because the absolute angular position is calculated using magnetic signals rather than optical ones, magnetic encoders are not affected by the presence of contaminating moisture or particulates in the way that optical encoders are. Magnetic encoders are therefore much better suited to harsh industrial environments.
There are two main types of magnetic rotary encoder: on-axis and off-axis, with some devices are a combination of the two. On-axis magnetic rotary encoders comprise a single magnet, polarised transverse to the rotational axis of the rotary encoder, and a magnetic sensor arrangement arranged on the rotational axis of the encoder. An example of such an approach is described in U.S. Pat. No. 7,317,313 B2. However, such devices have a relatively low resolution and the layout of the components precludes the possibility of the encoder having a hollow shaft suitable for passing wiring through. Higher resolution rotary encoders use two magnetic fields, such as the rotary encoder of U.S. Pat. No. 8,760,153 B2, which incorporates an off-axis magnetised element in addition to the on-axis magnet. The magnetised element of that invention is a ring arranged coaxially with the rotational axis of the encoder. A circular magnetic track comprising a number of magnetic pole pairs is arranged coaxially with the rotational axis on a surface of the magnetised element.
Although addressing the problem of low resolution, wiring still cannot be passed through the shaft of the encoder due to the presence of the on-axis magnet. Entirely off-axis magnetic rotary encoders overcome this problem by allowing for a hole in the centre of the magnetised element. However, in the absence of an on-axis magnet, multiple off-axis magnetic tracks are needed in order to achieve a high resolution. Off-axis magnetic rotary encoders therefore comprise a magnetised element with multiple magnetic tracks, each magnetic track comprising a number of magnetic pole pairs. The multiple magnetic tracks are arranged either on those surfaces of the magnetised element parallel to the rotational axis of the encoder, as with the encoder of U.S. Pat. No. 8,358,124 B2, or on those surfaces of the magnetised element perpendicular to the rotational axis, as with the encoder of U.S. Pat. No. 7,999,536 B2.
High resolution off-axis encoders allow for the possibility of a hollow shaft through which wiring can be passed. However, a gap must be maintained between each of the multiple magnetic tracks in order to prevent interference between the magnetic fields of the individual magnetic tracks. Consequently, the use of multiple magnetic tracks increases the thickness or width of the magnetised element of the encoder, and thus increases the overall size of the encoder. There is clearly a need for a reliable magnetic rotary encoder with a sufficiently high resolution and a compact structure.